1. Field of the Invention
The present invention relates to a method and apparatus for the temperature balancing control of a plurality of heat exchangers.
2. Description of the Prior Art
As various plants become larger in size, various heat exchangers for use in them ought to be enlarged correspondingly. It is the actual situation, however, that the enlargement of the heat exchangers is limited in relation to manufacturing equipment and fabricating techniques.
For this reason, in a large-sized plant, the case of using a plurality of heat exchangers connected in parallel by piping is increasing. On that occasion, the control of the distribution of fluid flow rates to the respective heat exchangers becomes a problem.
More specifically, even when the respective heat exchangers are fabricated in accordance with the same specifications, the dispersion of fluid resistances is inevitable, and dispersions arise also in the fluid resistances of pipes connecting the heat exchangers, the fluid resistances of valves disposed midway of pipes, etc. Therefore, the flow rate distribution to the individual heat exchangers becomes unbalanced, with the result that unbalanced temperatures develop in various parts of the heat exchangers.
It is necessary to correct the unbalance and to operate all the parallel heat exchangers while their temperatures are being balanced.
The temperature balancing control is performed by equipping the respective heat exchangers with control valves for regulating the fluid flow rates and regulating the control valves individually. When only the temperature balance is considered, the temperatures may be balanced with all the control valves kept close to their fully closed states. In order to realize the stable operation and efficient operation of the plant, however, the temperatures should preferably be balanced with the control valves kept close to their fully open states.
A known prior-art method for the temperature control of a plurality of heat exchangers is disclosed in the official gazette of Japanese Patent Application Publication No. 51-30304.
In the aforementioned known temperature control method for a multiple heat exchanger in which a plurality of heat exchangers are arranged in parallel, temperatures are sensed at the same positions of the respective heat exchangers except for the inlets thereof for a fluid subject to heat exchange, the mean temperature of the sensed temperatures is evaluated, and the sensed temperatures are compared with the mean temperature so as to regulate the flow rates of a heat exchanging fluid, whereby the temperatures of the fluid subject to the heat exchange are averaged.
With this known method, the flow rates of the heat exchanging fluid in the respective heat exchangers are controlled using the mean temperature as a reference value. It is theoretically possible, however, that the balanced relationship of the temperatures holds in the state in which the openings of all control valves for controlling the flow rates are close to the full opening or the full closure. Therefore, the method left intact is problematic in practical use.
In addition, a prior-art control method according to which the temperatures do not become balanced in the full closure direction is disclosed in the official gazette of Japanese Patent Application Publication No. 58-9920.
In a multiple heat exchanger wherein a plurality of heat exchangers are used in parallel, this method consists in sensing the temperatures of the same positions of the respective heat exchangers except for the inlets thereof for a fluid subject to heat exchange and the inlets thereof for a heating fluid, selecting the temperature of any desired one of the positions as a control reference value, and adjusting the fluid flow rates of the respective heat exchangers so that the sensed temperatures may agree with the control reference value.
In such method, using the desired position for the control reference value, the fluid flow rates of the respective heat exchangers are adjusted so that the sensed temperatures may agree. However, when control valves have become fully open, they cannot be opened more, and the method becomes uncontrollable.